The present invention relates to a scanning optical system employed, for example, in a color laser beam printer, color laser copier or the like.
Color printers have been developed and widely used for an output device of computers, color copiers or the like. In order to form a color image at a high speed, a color laser printer is widely employed. The color laser beam printer typically includes a plurality of photoconductive drums, and corona chargers, developing units etc. for respective color components, i.e., yellow (Y), magenta (M), cyan (C) and black (B).
In order to form electrostatic latent images consisting of the four color components on the photoconductive drums, respectively, four scanning optical systems, each of which has a polygonal mirror and imaging optical system, may be used. However, such an apparatus provided with four polygonal mirrors and four imaging optical systems for four photoconductive drums costs a lot in manufacturing thereof. Recently, an apparatus making use of a single polygonal mirror for simultaneously deflecting four beams corresponding to the four color components has been developed. The four deflected beams are incident on four imaging optical systems, respectively, thereby the four beams being directed to the four photoconductive drums. Thus, four image components can be formed simultaneously.
In order to simultaneously deflect a plurality of laser beams using a single polygonal mirror, in particular, to make a plurality of beams be incident on the plurality of imaging optical systems arranged separately in the auxiliary scanning direction, respectively, the beams should be inclined at different angles with respect to a plane perpendicular to a rotational axis of the polygonal mirror, or the beams are separated from each other in the auxiliary scanning direction.
If laser beams are inclined with respect to a plane perpendicular to the rotation axis of the polygonal mirror, the scanning lines, which are loci formed by moving beam spots on the surfaces to be scanned, curve in the auxiliary scanning direction (i.e., in a circumferential direction of photoconductive drums). By appropriately arranging a lens, which is one of the lenses of the imaging lens, having a power to converge a beam in the auxiliary scanning direction and located closer to the surface to be scanned than the other lenses, the curvature of the scanning lines in the auxiliary scanning direction, or the bow can be suppressed by a certain degree. However, the above-described lens should be manufactured to have a relatively wide effective area. Further, in view of the aberration compensation, it is preferable that the refraction surfaces have asymmetrical surface. Therefore, manufacturing such a lens is very difficult. Further, four of such lenses should be prepared, which increases the manufacturing cost of the entire scanning optical system.
Further, there is a requirement, among manufacturers, that if a certain accuracy is to be maintained the shape of the lenses should be the same to suppress the manufacturing cost.
The present invention is advantageous in that, a scanning optical system is provided, in which a plurality of beams incident on a single polygonal mirror are inclined with respect to a plane perpendicular to the rotation axis of the polygonal mirror, and the bow of the scanning lines can be well suppressed using inexpensive lenses.
According to the present invention, there is provided a scanning optical system that emits at least two beams to the same number of surfaces to be scanned, respectively. The scanning optical system includes a polygonal mirror that is rotated about a rotation axis thereof to deflect the at least two beams simultaneously to scan in a main scanning direction, the at least two beams incident on the polygonal mirror along the same number of optical paths, respectively, the same number of optical paths being arranged symmetrically with respect to a plane perpendicular to the rotation axis, and an imaging optical system that converges the at least two beams deflected by the polygonal mirror on the same number of surfaces to be scanned, respectively.
Such an imaging optical system includes at least two lenses, each of the at least two lenses converging an incident beam, in an auxiliary scanning direction which is perpendicular to the main scanning direction, on a corresponding surface to be scanned. The at least two beams deflected by the polygonal mirror pass through the at least two lenses, respectively. The at least two lenses have the same shapes, and the at least two lenses are arranged symmetrically with respect to the plane. An orientation of one of the at least two lenses is oriented oppositely, in the auxiliary scanning direction, to the other one of the at least two lenses.
With the above configuration, the pair of laser beams pass through equivalent portions of two lenses, which are arranged symmetrically with respect to the plane perpendicular to the rotation axis of the polygonal mirror. If the lenses are formed to appropriately compensate for aberrations for one of the pair of beams, since one lens is oriented oppositely to the other lens, aberrations for the other beam can also be compensated.
As above, by arranging a pair of optical paths symmetrically with respect to the plane perpendicular to the rotation axis of the polygonal mirror, the shapes of a pair of lenses corresponding to the pair of beams can be made identical. Accordingly, the same lenses can be used in the scanning optical system, which suppresses the manufacturing cost thereof.
Optionally, the two laser beams may intersect, in the auxiliary scanning direction, with each other at a point in the vicinity of a reflection surface of the polygonal mirror.
With this configuration, the thickness of the polygonal mirror, in the auxiliary scanning direction, can be reduced. As a result, the size of the polygonal mirror can be reduced, and therefore, a motor having a smaller power can be employed for driving the polygonal mirror.
Optionally, the at least two laser beams may intersect with each other at a point, within the plane perpendicular to the rotational axis of the polygonal mirror, in the vicinity of a reflection surface of the polygonal mirror.
Further optionally, the imaging optical system may include a lens group located on an upstream side of the pair of lenses, all of the at least two beams passing through the lens group.
In this case, an optical axis of the lens group may be parallel with the plane, the lens group being arranged such that an optical axis of the lens group is parallel with the plane and is located on a second plane passing through a point at which the at least two beams intersect with each other A shape of the lens group is configured to be symmetrical with respect to a main scanning plane which is a plane including the optical axis of the lens group and perpendicular to the rotational axis of the polygonal mirror.
Still optionally, each of the at least two lenses may be configured to be symmetrical with respect to a plane which includes a reference axis of each of the at least two lenses and parallel with the rotational axis of the polygonal mirror.
In this case, each of the at least two lenses may have an anamorphic aspherical surface expressed with two-dimensional polynomial, inclination of the anamorphic aspherical surface varying in the auxiliary scanning direction.
Optionally, the at least two lenses may be formed using the same molding.
According to another aspect, there is provided a scanning optical system that emits at least two beams to the same number of surfaces to be scanned, respectively, which is provided with a polygonal mirror that is rotated about a rotation axis thereof to deflect the at least two beams simultaneously to scan in a main scanning direction, the at least two beams incident on the polygonal mirror along the same number of optical paths, respectively, the same number of optical paths being arranged symmetrically with respect to a plane perpendicular to the rotation axis, and an imaging optical system that converges the at least two beams deflected by the polygonal mirror on the same number of surfaces to be scanned, respectively.
In such an imaging optical system, further provided is a same number of lenses, each of the same number of lenses converging an incident beam, in an auxiliary scanning direction which is perpendicular to the main scanning direction, on a corresponding surface to be scanned The at least two beams deflected by the polygonal mirror passing through the same number of lenses, respectively. The same number of lenses have the same shapes, and are arranged symmetrically with respect to the plane An orientation of one of the lenses receiving a beam proceeding along an optical path arranged on one side of the plane being oriented oppositely, in the auxiliary scanning direction, to another lens receiving a beam proceeding along an optical path arranged on the other side of the plane.
According to further aspect of the invention, there is provided a scanning optical system that emits an even number of beams to the same number of surfaces to be scanned, respectively, which is provided with a polygonal mirror that is rotated about a rotation axis thereof to deflect the even number of beams simultaneously to scan in a main scanning direction, the even number of beams incident on the polygonal mirror along a same number of optical paths, respectively, the same number of optical paths being arranged symmetrically with respect to a plane perpendicular to the rotation axis, and an imaging optical system that converges the even number of beams deflected by the polygonal mirror on the same number of surfaces to be scanned, respectively.
The imaging optical system may include a same number of lenses, each of the same number of lenses converging an incident beam, in an auxiliary scanning direction which is perpendicular to the main scanning direction, on a corresponding surface to be scanned, the even number of beams deflected by the polygonal mirror passing through the same number of lenses, respectively. The same number of lenses have the same shapes, and the same number of lenses being arranged symmetrically with respect to the plane. An orientation of lenses of the same number of lenses receiving beams proceeding along optical paths arranged on one side of the plane are oriented oppositely, in the auxiliary scanning direction, to the other lenses receiving beams proceeding along optical paths arranged on the other side of the plane.